A simple reversed-phase high-performance liquid chromatographic (RP-HPLC) method based on isocratic elution has been developed and validated for the simultaneous quantitation of flavonols (myricetin, quercetin, kaempferol, and isorhamnetin), flavones (luteolin and apigenin), and phenolic acids (chlorogenic, caffeic, ellagic, and rosmarinic acids) as important constituents in fruits, vegetables, and medicinal plants. Analysis was achieved on a C18 column at ambient temperature. The wavelengths used for the detection of flavonols, flavones, and phenolic acids were 370, 350, and 325 nm, respectively. After acid hydrolysis, the flavonoid aglycones were quantified straightforwardly in 20 dry herbal samples. The plants with the highest flavonoids were Rosa damascena, Solidago virgaurea, Ginkgo biloba, and Camellia sinensis. The contents of flavonoids aglycons ranged from 0.54 to 11.10 mg/g, from 0.03 to 14.80 mg/g, from 0.19 to 2.76 mg/g, from 0.15 to 2.36 mg/g, from 0.27 to 2.05 mg/g, and from 0.42 to 1.82 mg/g for quercetin, kaempferol, isorhamnetin, luteolin, apigenin, and myricetin in dry plant samples, respectively.